Exceptionally clear and contrasting behavioral defects have been found in adult Syrian hamsters after specific lesions of visual cortex (impairment of pattern discrimination) and of the superior colliculus (impairment of visually guided orientation). However, if either type of brain damage is inflicted in the neonate rather than in the adult, the behavioral defect, examined at maturity, is often much less severe or even absent. Our approach to explaining the basis for these differences in the effects of early vs. late lesions depends on neuroanatomical and functional studies: (1) Experimental neuroanatomical studies have revealed anomalies in the central projections of the retina after early lesions of either type, most dramatically after early lesions of the colliculus. We have proposed certain factors causing the formation of such altered connections, e.g., competition for terminal space and compensatory sprouting, and we have designed further experiments to test variations and generality of these factors. We will systematically examine changes in the plasticity phenomena with age, and undertake studies of normal and abnormal development of the relevant axons in the neonate hamster. (2) Studies of visual function in hamsters with early lesions have begun to reveal that the neuroanatomical anomalies are a mixed blessing: both adaptive and maladaptive effects are seen. We will employ videotape analysis of visually elicited orientation movements in animals with known anomalies of central connections, for comparison with normals.